In recent years, a lithium-ion battery and a lithium-ion capacitor respectively having a high battery voltage and high energy density receive attention from the standpoint of an energy storage system noting renewable energy and from the standpoint of development of personal computers, cameras, mobile equipment and the like, and research and development thereof is actively progressed. As these negative electrode active materials, a carbon material is commonly used for the reasons that lithium can be absorbed/released, dendrite-like lithium is hardly deposited and safety is high, and a capacity is relatively high and excellent cycle characteristics are shown.
Then, in order to respond to request asking a higher capacity of a negative electrode of recent years, various investigations have been made, and for example, a porous carbon material is proposed in which a contact area between the electrolyte and the negative electrode active material is increased to decrease an internal resistance of a battery and thereby a charge-discharge capacity is increased, and a contact area with the electrolyte is increased to realize a high rate characteristics (Patent Document 1).
On the other hand, in order to achieve a higher capacity of the lithium secondary battery, a method of using an elemental metal such as Si, Sn, Al or the like, an alloy or oxides thereof is proposed. For example, while in a negative electrode material made of graphite, a capacity is limited to a theoretical capacity of 372 mAh/g, a negative electrode material, such as Si, Sn, Al or the like, which is alloyed with lithium, particularly Si, has a theoretical capacity of 4200 mAh/g. Therefore, it is said that a capacity can be increased infinitely by adding these negative electrode materials. However, these negative electrode materials have a problem that since the negative electrode material involves large changes in volume during charge-discharge, this causes cracks and exfoliation of the negative electrode active material and the collapse of the negative electrode to shorten a cycle life of charge-discharge.
Thus, a method of preventing the collapse by forming voids within the negative electrode material containing silicon oxide and a carbon material is proposed on the above-mentioned problem (Patent Document 2). The voids are formed by volatilizing a polymer having dispersed during carbonization.